As the size of the hull of a seagoing vessel increases so does inevitably the flexibility and the elasticity of this hull. Thus in a large freighter or tanker it is possible for the one end of the hull to twist several degrees relative to the other end, and for the hull to change in overall length by a measurable distance. Such twisting and deformation is due to the considerable force exerted over the length of the hull by the sea, and the changes in length are also caused by thermal expansion and contraction.
It is known to power such a boat with a plurality of drive motors. For example it is known to use a pair of intermediate-speed Diesel engines each having a rating of 40,000 HP to drive such a boat. These engines have parallel drive shafts which are spaced apart by a distance of more than four meters. Thus if there is a change in distance between these two motors, such as caused by thermal expansion or contraction, of 0.1 percent these shafts will move four millimeters relative to each other.
As a general rule the drive shafts are each fitted with a small-diameter pinion. Both of these pinions mesh with a single large-diameter drive gear which is carried directly on the propeller shaft of the ship. Thus any displacement of the shafts relative to each other will cause the gear teeth of the pinions and of the drive gear to move out of proper mesh, thereby rapidly wearing any or all of these gears. In a common type of deformation the drive axes of the two drive shafts no longer remain parallel, thereby twisting the pinions relative to the drive gear and rapidly damaging these gears.